: The investigator will test the hypothesis that CTLA-4 downregulates T-cell activation and is normally involved in the prevention of autoimmunity. Evidence for this premise comes from the investigator's finding that the CTLA-4-deficient mice develop splenomegaly and lymphadenopathy, multi-organ lymphocytic infiltration and tissue destruction with severe myocarditis and pancreatitis, and die by 3-4 weeks of age. Three specific aims will be tested. The role of CTLA-4 in regulation of T-cell dependent immune responses will be analyzed by determining how CTLA-4 deficiency affects T-cell responses in vitro and whether CTLA-4 mediates its inhibitory effects solely through interactions with B7-1 and B7-2. The mechanisms of CTLA-4-mediated inhibition will be studied using naive and activated T-cells from CTLA-4-deficient, TCR transgenic mice. Emphasis will be placed on susceptibility to apoptosis and lymphokine production. The contribution of CTLA-4 to systemic autoimmune responses will be addressed by assessment of the pathology of in CTLA-4-deficient mice, determination of whether self-reactive T-cells appear in CTLA-4-deficient mice, and whether the course of EAE in MBP-specific TCR transgenic mice is altered when these mice lack CTLA-4.